Need help building carputer with HU!

I was hoping to just run a cheap carputer as just a mp3 storage device for my audio setup in my car. But I also want to use a head unit for more than that. I also want to use the hu for volume and xm and running regular cds. But I want my audio to be as good as possible as well. So I have no idea what to get to make this work. I want to be able to have 4.1 audio in case I do watch a movie or something in my car and to be able to pump out massive sound for my mp3's. So could you suggest me some parts. Like a good Hu, amp, subs, speakers, or whatever else I may need. I jsut have not been able to find exactly the answer I am looking for on the forums. Ive been researching what to get for a while but I am totally lost. Also BTW I have an 88 saab 9000 turbo for a car.

get the clarion Eq it has 1 aux input,3 amp inputs ,front and rear fade,volume control and sub control plus it gives out 7volts to all of your amps. i have one with my dvd hookup. It's great when i watch movies with police sirens i look all around for cops. lol paid aboy 60.00 for it i know you can find it cheaper.